Is there any particle physics beyond the standard model?

“…this consensus has been brought about, not by shifts in philosophical preference or by the influence of astrophysical mandarins, but by the pressure of empirical data.” –Steven Weinberg

One of the most fundamental questions we could ever ask about all of existence is “What makes up the Universe?”

Image credit: Misti Mountain Observatory.

I don’t mean “stars and galaxies,” like you see above. That might make up the Universe on the largest scales, but that’s taking a look at the question of what the fundamental constituents of the Universe compose themselves into.

The other side of the coin is to look at ever smaller and smaller scales at matter, and try to figure out what the smallest, indivisible things are. To take anything, whether it’s a supercluster, star, planet, human or amoeba, and break it down into the smallest things possible.

Image credit: PNAS, December 26, 2007, vol. 104, no. 52, 20719-20724.

Beyond the cellular level, beneath the molecules and atoms and protons of existence, we can finally reach the fundamental, indivisible particles that cannot be split into anything smaller. These particles — the quarks and gluons, the charged leptons and neutrinos, the electroweak gauge bosons and the Higgs — make up the Standard Model of elementary particles. Combined with gravity, as far as we can tell, these are the the most fundamental constituents of all the normal* matter and energy in the Universe.

Image credit: Fermilab, modified by me.

We learn about these particles — their existence, charges, masses, and (for the unstable ones) their lifetimes and decay properties — not from building ever-more-powerful microscopes. Instead, we build ever-more-powerful particle accelerators and colliders, and measure what comes out with gigantic detectors.

By accelerating particles as close as possible to the speed of light, and colliding them with either particles-or-antiparticles moving with an equal and opposite amount of momentum, we can use a giant fraction of that energy (up to 100%) to simply create new particles; anything allowable by the laws of physics.

Image credit: Maximilien Brice, © CERN.

You build your giant detectors around the collision point, and measure what comes out; this is how your modern particle accelerators work. Now, here’s the thing: you don’t make the vast majority of these fundamental particles — especially quarks and gluons — on their own.

Because of how long they give, they combine with other fundamental particles and form bound states, such as mesons or baryons, before they decay.

Image credit: The Aftermatter of

One of the variations of mesons that exist consist of a bottom quark (or antiquark) and a strange antiquark (or quark) pair. Both the bottom quark and the strange quark are unstable, and there are a variety of decay products that can arise from this particle.

Image credit: Fermilab Today, at

One of the decay channels — and it’s a rare one — is that this particle could decay into a muon and an anti-muon.

Now, here’s where it gets interesting: all fundamental particles that exist in the Universe couple to one another. The question of exactly how they couple and by exactly how much is what keeps many theorists in particle physics employed, because the underlying physics is known, but the calculations of things like event rates, branching fractions/ratios and decay parameters needs to be calculated. Moreover, if there is any interesting physics beyond the Standard Model, it will show up as a departure in the decay rates/ratios of particles such as this.

Image credit: David Straub, based on this paper:, via S. Carroll.

In other words, you can see what the Standard Model prediction is: about 3.5 × 10-9 of the bottom-strange mesons will decay into muon-antimuon pairs, and about 1.1 × 10-10 of the bottom-down mesons will decay into muon-antimuon pairs.

Any departure from this becomes very strong evidence for physics — and fundamental particles — that are outside of the Standard Model. However, a measurement of these branching ratios that lines up with the Standard Model will very severely constrain alternatives to the Standard Model, especially Supersymmetry (SUSY).

Image credit: DESY in Hamburg.

Basically, the lower-in-mass the (theoretical) superparticles are, the more they affect that rate. So if we measure the rate to be what the Standard Model tells us to be, we can constrain the mass of any (possible) SUSY particles to be very large, and the more we constrain that rate to be in agreement with what the Standard Model tells us, the worse it looks for supersymmetry as having any relevance for our Universe.

Well, guess what came out this week?

Image credit: LHCb, CERN, R. Aaij et al., from

The first actual measurement, rather than a constraint, of that decay!

And… do you want to know what they found? Whether we have evidence that the Standard Model is perfect, or whether there’s something new out there, on the brink of discovery?

Image credit: LHCb, CERN, R. Aaij et al., from

I’ve got to say, the results are in, and what they basically state is that there is no need for any physics beyond the Standard Model, as they measure rates that about 3.2 × 10-9 of the bottom-strange mesons (with error bars of about +1.5 and -1.2) will decay into muon-antimuon pairs, and less than 9.4 × 10-10 of the bottom-down mesons decay into muon-antimuon pairs, both the best constraints ever. (Full PDF here.)

To paraphrase a particle theorist friend of mine:

This constrains the masses of the supersymmetric particles to be anywhere from about 1 TeV up to infinity. I’m betting on infinity.

In other words, there is currently not a shred of experimental evidence in support of the need for — or existence of — SUSY.

Image credit: Geoff Brumfiel from Nature.

For those of you keeping score, no SUSY at all energies means no on the question of string theory, so don’t hold your breath on that front, either. We may have reached the end of what we can learn about the fundamentals of the Universe from particle physics, and the more the data from the LHC continues to agree with the Standard Model, the less attractive “new physics” theories like SUSY, extra dimensions and technicolor become.

The Standard Model still has problems, and there must be physics — at some point — beyond it, but this could be it for particle physics. Is there any particle physics beyond the standard model? So far, the evidence says no.

* – Dark matter not included.

51 thoughts on “Is there any particle physics beyond the standard model?

  1. For those of you who’ve seen Matt Strassler’s article on this:

    Yes, I am aware of it, and I think he’s thoroughly wrong. If SUSY doesn’t solve the hierarchy problem then there’s no motivation to have SUSY in particle physics. Dark Matter has many other perfectly good candidates and coupling-constant unification is both completely unnecessary and only an aesthetic bias.

    In other words, if there’s no SUSY at the LHC then there’s no longer any good reason to believe that SUSY exists at any energy.

  2. Wow.. I mean.. Uhm… I didn’t really understand any of that. But I get the feeling that what is reported on is incredibly important. But it won’t be until Minute Physics does a video about it that I will have some semblance of a clue as to what that importance might be. Maybe it’s like the alphabet: Okay, so now we know the fundamental building blocks of existence, but how long until we get to Shakespeare or Sagan? I guess that what I’m getting at (and guessing at my own intentions should illustrate how lost I am) is that we have finally graduated from elementary school; while we may know (possibly) that we have identified the fundamentals, we still have a long way to go before we get to the heights of literature. We have described the ground, and now it is time to reach for the sky (or at least the second floor). Am I anywhere close to understanding the intention of this article?

  3. If we want to be sure of inflation, then there is still a question about inflaton particle and the associated field. And DM is really a bugger as far as SM.

    But at the energies we’re probing, it might very well be there is nothing more.

  4. Call me when it’s at 5-sigma.

    Just kidding, no need to call. I’ll be watching, as this is intensely interesting.

  5. Excellent research, though I couldn’t understand a word in that paper.

    “this could be it for particle physics. Is there any particle physics beyond the standard model? So far, the evidence says no.” Umm, the devil is always in the details. The standard model is indeed amazing; but the search for new particles isn’t dead yet. There is still so much to explore and understand.

    These details are important. There is not a satisfactory theory of
    – quantum gravity
    – neutrino mass
    – dark matter
    – baryon asymmetry

    Unsolved problems in particle physics, Sergey Troitsky, Dec 2011

  6. Awesome!

    Now that we have dropped the idea of hidden dimensions and the rest of SUSY, can we finally get some people to focus on the DM enigma? Maybe an upstanding gent who will admit our math is probably wrong and eliminate that garbage once and for all?

  7. Didn’t Feynman say that eliminating symetries had led to some of the greatest advances in physics? The implication (for me) being that SUSY calculations are not incorrect, only incomplete.
    Please excuse my undergrad level of understanding.

  8. A great metaphyscal mistake of any science is to assume that it is complete. However complete any science may seem; it is only a fragment of the whole story. It of course is tempting to overlook details, insignificant exceptions, unrelated problems, “unanswerable” questions and declare the end of such and such science and that such and such model of some area of science is complete. But the universe is much more complex than any of our puny human toy models.

    In the extremes of science (whether the tiniest, the largest or somewhere in the middle); we should not be surprised when we find something fundamentally new that redefines everything that we thought we knew. Herein lies the profound necessity of ceaseless curiosity and systematic observation of our world.

  9. “A great metaphyscal mistake of any science is to assume that it is complete. ”

    A great problem with those looking on is they think that scientists assume that science is complete.

    If they claim something with all the caveats, they’re ignored, called wishy-washy and someone who claims “PROOF!!!!” is listened to even though they’re an obvious quack.

    But if they get a spokesman to lay out the facts, they’re derided for hubris and saying that there is no uncertainty and that there is no debate.

    And assumptions like yours is one of your own construction.

  10. Well, you’ve just set up the stage to make another great post: now that SUSY is crumbling, what are the alternatives to dark matter? As far as I knew, we _need_ a new particle to explain DM, because neutrinos don’t suffice.

  11. These arguments by Ethan are flawed. The latest data on this decay, weaken the bounds on susy. So what is he talking about?
    Personally, I doubt susy exists at low energies, but who cares about people’s opinions, lets talk about facts. The data weaken the bound on susy, not the other way around.
    Furthermore, susy is motivated by dark matter, unification, stability of the ew vacuum, quantum gravity, and so on. And none of these motivations have gone away.
    Again, I am not advocating susy here, I am just interesting in facts and evidence. Rather than bias and spin, which Ethan seems to be advocating here.

  12. Bob, this data strengthens the bounds on SUSY. It makes the parameter of workable (as in ‘agrees with experiment’) SUSY theories smaller.

    And yes SUSY was motivated to solve a bunch of problems. The thing is not all SUSY theories solve those problems, e.g. beyond a certain smallest SUSY particle mass it no longer provides any good DM candidates. If we push the areas where SUSY could exist to the point where it no longer can explain what it was invented to explain, what exactly is the motivation for believing it may exist?

    That’s what Ethan is talking about.

  13. CB, that is wrong. The latest data actually weaken the bounds on SUSY. Have you looked at the actual papers? Have you looked at the updated constraints on SUSY models? If you read the papers, you will see that the bounds on SUSY are now roughly the same, if anything, they have slightly weakened.
    Again I am not advocating SUSY; I am just stating facts, rather than bias and spin.

  14. Bob, you’re referring to the 95% confidence upper limit being higher in this study than previously? Yes, you’re right, that means SUSY is less constrained by these interim results. I think Ethan, and certainly myself, are more interested in the average value which is spot on with the SM prediction. What this really means is going to depend on what happens to that number as more data is processed. If it tightens around the current average, that’s bad for SUSY. If it drifts towards the upper limit, that’s good for SUSY. If you believe the current data is a good indication of anything, *probably* it’s going to stick near the current mean and the edges of the curve are going to pull in, and SUSY will be severely constrained.

    But like I said before, call me when we’re at 5-sigma. 🙂

  15. CB, glad you agree that these latest results weaken the bounds on SUSY; which is the exact opposite of what Ethan claimed.

  16. Can anyone explain to me please why the following explanation for dark matter is not plausible?

    “About 70% of the universe is occupied Type III civilisations who are capable of hiding everything but gravity from us.”

  17. Dude, I’m only a human from planet earth, and you’re asking me how to cloak everything but gravity from potential interstellar observers?

    I’m an astrophysics drop-out, but if I recall correctly, we know all that we know about space beyond our solar system by observing em radiation (and smashing a few hadrons and leptons together).
    I’ve seen a guy in a van rigged up with cameras and plasma screens to a reasonable job of driving around ‘invisibly’.

    Given that there is time enough for a civilisation elsewhere to have reached out level of technology several billion years ago, then with a small but still unscientific leap of imagination, could they not do something similar with all em radiation that should bounce off them and hit our extra large telescopes?

    Let’s keep it open as a hypothesis anyway, for want of any evidence to the contrary.

  18. No, dude, I’m asking what, if you can find anything, is plausible about your statement.

    The problem is there isn’t anything.

    There isn;t anything plausible about “Oh, maybe it’s a Type 3 civilisation who are hiding everything but their gravitational mass, Oh, and by the way, they weigh vastly more than the rest of the entire universe!”.

  19. Do you find the idea that despite trying to hide from us, they can’t hide their gravitational pull (which they would definitely know would be much more than the mass of the entire galaxy that isn’t them), but decide to hide anyway plausible?

  20. And remember, each answer to the question “If yes, what are the chances?” that are less than 100% reduces the plausiblity of the entire edifice.

    Indeed, this is the entire raison d’etre of Occam’s razor.

  21. If they ‘eat’ stars then they may assume, if they are wise, that there are others out there who can also ‘eat’ stars.

    For the same reason a dog buries bones, to save it for later while hiding it from other dogs.

    Hiding a star is probably a lot cheaper, energetically speaking, than eating a star.

    Maybe it takes half the energy gained from a star to build the machinery needed to eat another star, but only a comparatively small amount of energy is needed to hide a star from potential rival predators.

    Clearly, being only human, I am not going to know how star eating and star hiding technologies work, but I do know how competition for food works in nature.

    That’s my cut, thanks to Occam’s razor.

  22. No I don’t think they are hiding from ‘us’, that’ll be you’re anthropic bias. We are not the centre of the universe, but you should know that!

  23. Yup more “if”.

    Each “if” is a non-certainty. Indeed for all your clauses the certainty is astronimically small.

    Therefore the multiplication of each miniscule chance with each other makes the entirety implausible to the extreme.

    It seems you think you shave with occam’s razor.

    I note you have not one reply to the question “what is plausible about your scenario”.

    That is because it is entirely impausible to the point of being ridiculous.

    Your ideas are complete bollocks and there’s a thread for complete bollocks you can go point on.

    Go there and stay off the sane parts of the site.

  24. “No I don’t think they are hiding from ‘us’, that’ll be you’re anthropic bias.”

    Things that don’t exist can’t try things, therefore this is correct.

    Your woometer is pulling you toward idiocy. Pull back while you may still have a chance.

  25. Are you the moderator? If not, (and that’s a social ‘if, not a physics ‘if’) then you are saying that you can spot bollocks but the moderator cannot.

    What the hell is a woometer? Are you in need of an ego boost? If you reduce mine, that doesn’t make yours bigger.

    I think you’ve been caught off-guard by the hypothesis and you’re acting classic scared and defensive.

    Astronimically small? Yes, the names of stars are small, usually just a few digits and a couple of letters. Or was that a Type-O? or a Type-I?

    You’re playing with semantics, and spouting neologisms, and have absolutely no science with which to defeat me

  26. The saddest thing is not that you probably have the knowledge to refute my ‘attempt to look smart’ without resorting to insults, the saddest thing is that I would be so happy to learn the physics which refutes my attempt to look smart, or was it an attempt to be helpful?

    Sometimes, unsolved problems are given new direction by total outsiders, and sometimes mistakes lead to new inspiration.

    Did you hear about the guy who suffered from a particular medical condition. No one knew the cause of this condition, but many had suggestions for how to treat it?

    So the guy was typing one of the suggestions into his blog, and he mistyped the name of the drug he wanted information on. So:

    [drug] for [condition] was mistyped [drog] for [condition] and the spellchecker suggested [dog] for [condition].

    Amused by this, the chap put it out, for a laugh that the condition may be caused by dogs. Some years later, we all now know that the condition is, in fact, caused by dogs.

    The scientists who had spent all their life searching for the cause of the condition still do not believe that it is caused by dogs, but they’re just being sour grapes.

    If you wish to insult my way of thinking and communicating, the label for me is ‘autistic’ not ‘moron’.

  27. If you want to do all that, go to the thread that you get to do that on and stop cluttering up other threads.

    Here, once again, is the link:

    but remember the Jefferson quote. So far you’ve failed to make a statement that even gets to “plain wrong”. All you’ve done is babble and plot if, if, if, if, if and ignored anything other than empty whatifferey.

    Your ideas are not even enough to make sense, never mind be tested or refuted.

    And that’s why your posts are in the wrong place.

  28. No, that’s not correct. ‘Moron’ is a now disused term for someone with mental retardation.
    I am diagnosed with an autistic spectrum condition.

    Anyway, I certainly won’t be back to this thread, because you’re brought a bad smell here, so no-one else will comment, and I don’t really like you, whoever you are?

  29. Of course, there is. THE STANDARD MODEL CANNOT BE THE LAST WORD BECAUSE THERE ARE A NUMBER OF QUESTIONS THAT IT DOES NOT ANSWER. For example, four very important open questions are the proton spin puzzle, the EMC effect, the distributions of electric charges inside the nucleons as found by Hofstadter in 1956, and the ad hoc CKM matrix elements. Please, take a look at the following papers for a deeper understanding of the matter (all are available by means of Google): 1) Weak decays of hadrons reveal compositeness of quarks; 2) The Higgs-like bosons and quark compositeness; 3) The Higgs-like bosons couplings to quarks .

  30. really interesting, but when I read the sentence …”indivisible particles that cannot be split into anything smaller” …right at the beginning of an article, I know that what follows is maybe amazing science information, but every time someone says, …this is it, we found the smallest thing possible, I know something is wrong.

    People should really listen more to Nassim Haramein and his thesis, even if he is not 100% right opens a new understanding.

    However, what is described here is truly amazing and the question is, why shouldn´t it be possible to split something into anything smaller? Only because something can´t be split , or we are not able to do so, doesn´t mean it is the smallest “thing” …two different points!

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